The present invention relates to an image reading apparatus for reading a color manuscript (original) particularly.
For a manuscript reading apparatus such as a scanner, high resolving power, a high quality of image and others are demanded and for a manuscript reading apparatus such as a copying machine, high speed, correspondence to large width (A3 size) and the lifting of a manuscript in the binding margin of a book is demanded in addition to the above. Describing them in physical and optical terms, a high Nyquist frequency, a high modulation transfer function (MTF), the high sensitivity of a sensor, a large angle of view and the large depth of a focus are demanded. Of them, it is MTF and the depth of a focus that are related to the specifications of an imaging optical system after a sensor and conjugate length are determined.
Further, in the case of a color copying machine, in addition to the above conditions, there is also a problem related to the chromatic aberration of a lens. The chromatic aberration is classified into axial chromatic aberration equivalent to displacement in the direction of an optical axis and lateral chromatic aberration equivalent to displacement in the longitudinal direction of a charge-coupled (CCD) line sensor and have a different bad effect in the evaluation of an imaging optical system. Concretely, axial chromatic aberration displaces the peak of MTF of each color in an imaging optical system in a defocused direction, MTF is deteriorated as a result because balance is required to be kept and the quality of an image is deteriorated. In the meantime, as lateral chromatic aberration displaces the imaging position of each color in the longitudinal direction of a CCD sensor, chromatic displacement shown in FIG. 12B is caused through each color is to be overlapped as shown in FIG. 12A.
As for MTF characteristics, as aberration in monochrome except chromatic aberration also deteriorates MTF, its mechanism is very complicated. For evaluation items related to MTF, there are the height of MTF at a level of a CCD sensor, tolerance (the depth of a focus) in case a manuscript is lifted and color balance (xcex94MTF) in each angle of view.
Of the MTF evaluation items, the increase of xcex94MTF and the increase of lateral chromatic aberration are different in a physical (optical) behavior, however, from the viewpoint of output from a CCD sensor, they produce similar effect. That is, as shown in FIG. 12C, there occurs a phenomenon that signal values (levels) of three colors to be output from a pixel in the same position in the longitudinal direction vary and as a result, chromatic information acquired by synthesizing them also varies from an original value. The variation of chromatic information means that the color reproducibility based upon a manuscript of an output image is deteriorated in the case of a color copying machine. In the case of a color copying machine, processing for converting the read information of BGR to yellow (Y), magenta (M), cyan (C) and black (K) which are respectively the color of toner is required, however, if the read chromatic information of original data includes displacement in unspecified quantity, correction is very difficult.
There is also a problem intrinsic to lateral chromatic aberration. It is pigmentation at an edge in the same chromatic area. If an image the contours of which are often obscure such as a photograph is copied and output, slight displacement of colors is often not an eyesore. However, if only an edge of a character and a graphic form in a vertical scanning direction has different color due to lateral chromatic aberration in a manuscript composed of a combination of characters and graphic forms such as a business document, it is very conspicuous and makes a bad impression on a user. In the meantime, in the case of xcex94MTF, color is different as a whole, however, it is never caused that only an edge has different color. At this point, lateral chromatic aberration has a worse effect upon the quality of an image than xcex94MTF.
Further, in a digital color copying machine, generally, different image processing is applied depending upon whether a manuscript is a text or an image such as a photograph. That is, if contrast between a text and a background is clear in case a manuscript is a text, visibility is satisfactory. Therefore, processing such as an edge part in a text is highlighted is executed. In the meantime, a case of an image manuscript is contrastive and if the gradation of color is smooth, visibility is satisfactory. As described above, the image processing section of a copying machine is often provided with a picture/character separating processing function to differentiate processing depending upon the type of a manuscript.
Also, in a digital color copying machine, if read color is black, a black judgment function is also provided in addition to the above picture/character separating processing function because more beautiful output can be acquired in a case that printout is acquired using black toner than in a case that black is acquired by overlapping Y, M and C. Therefore, if output from CCD is off from black due to the above aberration particularly in case a black character is separated from image information, an error of picture/character separating processing and an error of black judgment are simultaneously caused, a black character in a manuscript is colored and output in an obscure state and reducibility is very deteriorated. Particularly, recently, because of the development and popularization of a personal computer and a presentation document preparation tool, a full color manuscript including high definition character information overflows and the frequency of copying them is also increased. Therefore, the above problem becomes important.
Heretofore, as measures for these problems, generally, the balance of characteristics has been kept by determining parameters of a lens in the design of the lens so that aberration is as small as possible in addition to image processing. However, in the design of a lens, the characteristics of MTF have a tendency to be deteriorated when the correction of lateral chromatic aberration is tried. Then, anomalous dispersion glass effective as a countermeasure of chromatic aberration is considered to be used, however, there is a limit in improvement by improving the characteristics of only a lens. Anomalus dispersion glass requires three to five times as high costs as normal glass material.
In the design of a lens, a method of reducing an aperture or an angle of view is adopted to decrease aberration, however, an angle of view cannot be greatly varied because of constraints of manuscript size and space in a copying machine. Then, a method of reducing an aperture (darkening a lens) to decrease aberration is adopted, however, because of the signal-to-noise ratio (S/N) of a sensor, if a lens is darkened maintaining high speed and high resolving power, the luminous energy of a lamp is required to be increased to secure required luminous energy and power consumption is increased.
Normally, power acquired from the outlet of an office is 1.5 kVA, however, in the case of an electrophotographic copying machine, heat is utilized for fixing toner and a heater for generating heat requires very large power. Currently, a color electrophotographic copying machine requires power close to 1.5 kVA to read a manuscript at 400 dpi. Therefore, the reduction of power consumed to read a manuscript is desired. Further, as the area of a sensor pixel becomes smaller when resolving power is enhanced up to 600 dpi maintaining copying speed, the relative quantity of light received by a sensor becomes a half or less. As the quantity of data is increased, a video rate is also increased and as a result, as power is also increased, it is currently impossible to darken a lens in addition and increase the power of a lamp. Rather, to compensate the received quantity of light made a half or less, a lens is required to be lightened.
There are many reasons for designing so that it is as light as possible in addition. Recently, a monochrome copying machine uses a xenon lamp the power consumption of which is small in place of a halogen lamp which has been adopted for a lamp. Naturally, as a full color copying machine will also use a xenon lamp to save energy, design in consideration of a xenon lamp is required. However, the quantity of light which a xenon lamp can emit is limited and when the process speed of a copying machine, S/N of a CCD sensor and resolving power are determined, the brightness of a lens is also necessarily led based upon them.
Also, for a countermeasure for lateral chromatic aberration, technique to correct output from a CCD sensor electrically is also well-known, however, in that case, MTF as an electric signal is deteriorated and it has a bad effect upon the reproducibility of a character. As a result, though resolving power is enhanced, the quality of an image is not enhanced and even if resolving power is enhanced from 400 dpi to 600 dpi in specifications described in a catalog and the quality of an image seems higher, actually, a situation that the quality of an image is not enhanced may occur.
For example, in the Unexamined Japanese Patent Application Publication No. Hei 7-46610, technique for storing information related lateral chromatic aberration and skew aberration in an optical system such as a lens in a memory and correcting the lateral chromatic aberration or/and the skew aberration of an image imaged on an image sensing device such as CCD based upon the above information is disclosed. In the Unexamined Japanese Patent-Application Publication No. Hei 9-205523, technique for reducing lateral chromatic aberration by arranging a reflector provided with a first reflecting surface which reflects only light having a specific wavelength and a second reflecting surface curved in the longitudinal direction for reflecting light having a predetermined wavelength and transmitted through the first reflecting surface between a manuscript and an imaging lens is disclosed. Further, in the Unexamined Japanese-Patent Application Publication No. Hei 6-261236, technique for extracting a picture signal in a dead zone existing between pixels of an image sensing device and enhancing resolving power by arranging a glass plate between an imaging lens and the image sensing device such as CCD, periodically varying the angle with an optical path of the glass plate, extracting a picture signal when xc2xd pixel is off and a picture signal when xc2xd pixel is not off from the image sensing device according to time series and processing these two picture signals is disclosed.
The above problems can be improved up to a level at which a user does not perceive if various parameters for the reproducibility of color, particularly pigmentation at an edge of a character and others can be determined well in a range to some extent of the cost. However, specifications for imaging performance when picture/character separating processing and the judgment of a black character are to be executed satisfactorily are very severe such as the displacement of colors caused by lateral chromatic aberration should be within approximately 1 xcexcm and higher characteristic is also required for xcex94MTF. To realize these specifications by only a lens and well-known technology in the range of the realistic cost as a mass-produced product in the above prior art, the design reaches a limit and there is a problem that it is very difficult to realize the specifications.
The present invention is made in view of the above situations and the object is to provide an image reading apparatus wherein the imaging performance of an optical system can be enhanced at a low cost and picture/character separating processing and the judgment of a black character can be satisfactorily executed.
To solve the above problems, an invention disclosed in Aspect 1 is characterized in that irradiation means for irradiating a manuscript, imaging means for imaging reflected light from the manuscript irradiated by the above irradiation means, photoelectric conversion means for converting the reflected light imaged by the above imaging means to electric information and parallel displacement means inserted into an optical path between the above manuscript and the above photoelectric conversion means for displacing rays of the above reflected light by quantity different depending upon each wavelength in parallel so as to reduce difference in the maximum magnification between wavelengths of the above reflected rays so that the difference is smaller than at least the pixel of the photoelectric conversion means are provided.
According to the above invention, after difference in the maximum magnification between wavelengths of rays of reflected light is reduced so that the difference is smaller than at least the pixel of the above photoelectric conversion means by displacing the above reflected rays imaged by the imaging means in parallel by quantity different depending upon each wavelength by parallel displacement means, the rays are made incident on the photoelectric conversion means. Therefore, lateral chromatic aberration can be reduced to the extent that it is ignorable. As a result, the imaging performance of an optical system can be enhanced at a low cost, and picture/character separating processing and the judgment of a black character can be satisfactorily executed.
Besides, according to another aspect of the present invention, there is an image reading method comprising the steps of: irradiating a manuscript; imaging reflected light from the irradiated manuscript; converting reflected light imaged to electric information; and displacing rays of the reflected light by quantity different depending upon each wavelength in parallel so as to reduced difference in the maximum magnification between wavelengths of the reflected rays so that the difference is smaller than at least a pixel in the photoelectric converting step.